New features:

- Expanding a tree node in tree view now updates the chart
- Added context menu for tree view:
	a) Deleting a whole folder: shows a confirmation dialog, if user confirms deletion, the whole folder is deleted recursively.
	b) Opening windows explorer window in selected folder.
- Added icons for context menu

Known bugs:
- if user tries to double-click (expand) a folder and he has no permission to view it the app crashes. Fix: one if-else block in PieChart.